Stability analysis of transmission system with high penetration of distributed generation

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(1)Stability analysis of transmission systems with high penetration of distributed generation. PROEFSCHRIFT. ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof.dr.ir. J.T. Fokkema, voorzitter van het College voor Promoties, in het openbaar te verdedigen op donderdag 21 december 2006 om 10:00 uur. door. Muhamad REZA elektrotechnisch ingenieur geboren te Bandung, Indonesië..

(2) Dit proefschrift is goedgekeurd door de promotoren: Prof.ir. W. L. Kling Prof.ir. L. van der Sluis Samenstelling promotiecommissie: Rector Magnificus, Prof.ir. W. L. Kling, Prof.ir. L. van der Sluis, Prof.dr. J. A. Ferreira Prof.dr.ir. J. H. Blom Prof.ir. M. Antal Prof.dr.ir R. Belmans Prof.dr. M. J. O’Malley. voorzitter Technische Universiteit Delft, promotor Technische Universiteit Delft, promotor Technische Universiteit Delft Technische Universiteit Eindhoven Technische Universiteit Eindhoven (emeritus) Katholieke Universiteit Leuven, België University College Dublin, Ierland. This research has been performed within the framework of the research program ’Intelligent Power Systems’ that is supported financially by SenterNovem, an agency of the Dutch Ministry of Economic Affairs.. Stability analysis of transmission systems with high penetration of distributed generation. Dissertation at Delft University of Technology. c 2006 by M. Reza. Copyright ISBN: 91-628-7039-4 Cover: A miniature of transmission lines tower in Madurodam, The Hague, photographed and modified by Muhamad Reza, and used for this thesis with permission from Madurodam B.V..

(3) To Bapak, Ibu, Novi and Rifqi..

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(5) Summary Stability analysis of transmission systems with high penetration of distributed generation Nowadays, interest in generating electricity using decentralized generators of relatively small scale is increasing. Such generation is known as ’distributed generation’ (DG). Many of the prime movers of such DG technologies are based on renewable energy sources resulting in an environmentally-friendly power generation. It is well-known that the implementation of DG influences the technical aspects of the distribution grids. The impact of a small amount of DG connected to the grid on the power system transient stability has not been treated so often. When the penetration level of DG increases, its impact is no longer restricted to the distribution network but begins to influence the whole system. This work deals with the impact of implementing DG on the transmission system transient stability, with the emphasis on a potential transition from a ’vertical power system’ to a ’horizontal power system’ (Chapter 1). For this purpose, it is important to examine characteristics of DG that influence the dynamic stability behavior of a transmission system (Chapter 2). Therefore, DG units are classified based on the primary (both conventional and renewable) energy sources. To a large extent, the type of primary energy source determines the output power characteristics of DG and the type of grid connection applied. It also determines the utilization of power electronic interfaces. Based on the DG classification, basic models of DG technology to be used in the transient stability simulation of a large power system can be derived, presented in Chapter 3. A problem in power systems is maintaining synchronous operation of all (centralized) synchronous machines. The stability problem associated is called rotor angle stability. In this work, the impact of the DG implementation on this is investigated. Therefore, in Chapter 3, the phenomena of the rotor dynamics of synchronous machines, that determine the rotor angle stability of a power system, are explained by means of the swing equation, the power-angle curve and the equal area criterion concepts. Indicators for assessing the stability performance of a power system derived from these concepts are the maximum.

(6) ii. Summary rotor speed deviation and oscillation duration of the (centralized) synchronous machines. For simulation purposes, the widely known 39-bus New England dynamic test system is used with minor adjustments. The basic setup of the test system is described, while details are listed in Appendices B and C. Several software packages suitable for the transient dynamic simulation of DG in a large system are highlighted. The representations of different DG technologies within the software packages are discussed, with the emphasis on the representation of power electronic interfaced (converter connected) DG units. The investigation of the impact of a high DG (penetration) level on power system transient stability, is presented in Chapter 4. The impact of increasing DG penetration levels, DG grid-connection-strength, different DG technologies, and DG protection schemes of converter-connected DG are simulated and discussed. It is found that DG influences the system transient stability differently depending on the factors above. However, there is no significant stability problem observed up to about 30% DG penetration level regardless the technology. This is logical when all centralized generators remain in the system – as well as their active and their reactive power control and the inertia of their rotating masses – along with the increasing DG levels. Furthermore, implementing DG is a natural way of ‘limiting’ the power flows on the transmission lines. It improves the transient stability of a transmission system, since large power flows may have a detrimental effect on the damping of the oscillations: the heavier the lines are loaded, the weaker the coupling between generators and loads becomes, and the larger the oscillations of the centralized generators may be, especially with long lines. In Chapter 5, the investigation is focused on the impact of DG levels on the system transient stability when the increasing DG level is followed by a reduction of centralized generators in service resulting in a ’vertical to horizontal’ transformation of the power system. The emphasis is on the use of converter connected DG units to supply active power. Therefore, different from the preceding Chapter 4, the increasing DG level implies a reduction in rotating masses (inertia) and reactive power control ability in the system. In some cases, power system transient instabilities with very high DG levels (more than 50%) are found. Several solutions are proposed for these problems by means of rescheduling centralized generators and optimizing the power flow. In some cases, a minimum number of centralized generators has to remain in the system to avoid system instabilities. The results discussed in Chapters 4 and 5 are merely based on a deterministic approach, where the parameters of the test system are set to the typical values. Beside the deterministic approach, a stochastic analysis can also be used to study the transient stability of the power systems, as presented in Chapter 6. The study is focused on the impact of the stochastic behavior of DG. The results show that including the stochastic behavior of DG leads to a more complete and detailed view of the system performance. Finally, Chapter 7 investigates the situation when the power system is pushed towards a scenario, where DG penetration reaches a level that covers the total load of the original power system (100% DG level). The DG units are im-.

(7) Summary. iii. plemented via power-electronic converters within so called “active distribution systems” (ADS) connected to the transmission system also via power-electronic interfaces. The power system is still connected to a source that provides a constant 50 Hz voltage that is meant to give a (system) frequency reference for the generators but generates no power at steady state. Therefore, any power imbalance in the system must be compensated by generators in the ADS. However, due to the power-electronic interfaces, the output power of the DG units (and the ADS) are decoupled from the grid frequency. Therefore in this chapter, the voltage is used to detect and maintain the power balance. For this purpose, specific control concepts are developed. The simulation results show that by applying such control systems, the power balance in the power system can be maintained by the ADS. The research performed in this work indicates that from the transmission system stability point of view, if higher DG penetration levels are coming up, sufficient inertia and voltage support must be installed. Furthermore, one should be aware of the fact that the system behaves stochastically, especially with DG. To a certain extent regional balancing of power can be performed by local voltage control..

(8) iv. Summary.

(9) Samenvatting in het Nederlands Stabiliteitsanalyse van een transmissienet met een hoge penetratiegraad van decentrale opwekking Tegenwoordig neemt de interesse in het produceren van elektriciteit met relatief kleine decentrale eenheden toe. Deze manier van elektriciteitsopwekking wordt ook wel ‘distributed generation’ (DG) genoemd. Veel aandrijfsystemen van DG technologieën zijn gebaseerd op hernieuwbare energiebronnen wat resulteert in een milieuvriendelijke energieopwekking. Het is bekend dat de toepassing van DG de technische aspecten van de distributienetten be¨ınvloedt. De invloed van kleine hoeveelheden DG aangesloten op het net, op de transiënte stabiliteit van het transportnet is nog niet vaak onderzocht. Wanneer het penetratieniveau van DG stijgt, is het effect niet meer beperkt tot het distributienet, maar wordt het gehele systeem be¨ınvloed. Dit proefschrift onderzoekt de effecten van de toepassing van DG op de transiënte stabiliteit van het transmissienet, met de nadruk op een potentiële overgang van een ’verticaal gericht systeem’ naar een ’horizontaal gericht systeem’ (Hoofdstuk 1) Met dit doel is het belangrijk om de kenmerken van DG te onderzoeken, die het dynamische stabiliteitsgedrag van een transmissiesysteem be¨ınvloeden (Hoofdstuk 2). Daarom worden DG eenheden geclassificeerd op basis van de primaire energiebronnen (zowel conventionele als hernieuwbare). Het soort primaire energiebron bepaalt in grote mate de kenmerken van de energie-output van DG en het type netaansluiting dat wordt toegepast. Ook bepaalt het of vermogenelektronische interfaces worden gebruikt. Gebaseerd op de DG classificatie kunnen basismodellen afgeleid worden voor de DG technologieën om te gebruiken in de transiënte stabiliteitssimulatie van een transmissienet, wat in Hoofdstuk 3 behandeld wordt. Een probleem in elektriciteitsvoorzieningsystemen is het handhaven van synchrone werking van alle (centrale) synchrone machines. Het stabiliteitsprobleem dat daarmee samenhangt heet de rotorhoek stabiliteit. In dit proefschrift worden de effecten van de toepassing van DG daarop onderzocht. Daartoe.

(10) vi. Samenvatting. worden in Hoofdstuk 3, de fenomenen van de rotordynamica van een synchronische machine verklaard, die de rotorhoek stabiliteit van een transmissienet bepalen, middels de bewegingsvergelijking, de vermogen versus hoek curve en het gelijke oppervlakte criterium concept. De indicatoren voor het beoordelen van het stabiliteitsgedrag van een transmissienet afgeleid uit deze concepten zijn de maximumafwijking van de rotorsnelheid en de duur van de slingering van de synchrone machines. Voor simulatiedoeleinden wordt het bekende 39knooppunten New England dynamisch testsysteem gebruikt, met een aantal aanpassingen daarin. De basisopzet van het testsysteem wordt in dit hoofdstuk beschreven, terwijl de details worden gegeven in Bijlagen B en C. Verscheidene softwarepakketten, geschikt voor transiënte dynamische simulatie van DG in een groot transmissienet, worden behandeld. De representatie van de verscheidene DG technologieën in de softwarepakketten wordt besproken, waarbij de nadruk ligt op de weergave van DG eenheden met vermogenelektronische interfaces (converters). Het onderzoek van de invloed van een hoog (penetratie) niveau van DG op de transiënte stabiliteit van het transmissienet wordt in Hoofdstuk 4 uiteengezet. De invloed van de toename van het DG niveau, de sterkte van de DG netkoppeling, verschillende DG technologieën, en DG beveiligingsschema’s van via vermogenselektronica gekoppelde DG, zijn gesimuleerd en bediscussieerd. Er wordt aangetoond dat DG de transiënte stabiliteit van het transmissienet verschillend be¨ınvloedt afhankelijk van boven vermelde factoren. Echter, er is geen significant stabiliteitsprobleem gevonden tot de DG een 30% niveau bereikt, onafhankelijk van de DG technologie. Deze resultaten zijn logisch vanwege de nog steeds aanwezige centrale generatoren in het net, met de bijbehorende regeling van actief vermogen en blindvermogen en de inertie van de roterende massa, bij dit niveau van DG. Daarnaast is toepassing van DG een natuurlijke manier om de vermogenstransporten in de transmissielijnen te beperken. Dit verbetert de transiënte stabiliteit van het transmissienet, omdat grote vermogenstransporten een nadelige invloed hebben op de demping van de rotorslingeringen: hoe zwaarder de lijnen zijn belast des te zwakker de koppeling tussen de generatoren en de belastingen wordt en des te groter de slingeringen van de centrale generatoren kunnen worden, vooral bij lange lijnen. In Hoofdstuk 5, ligt de nadruk van het onderzoek op de invloed van het toenemende DG niveau op de transiënte stabiliteit van het net als dit samen gaat met de vermindering van in bedrijf zijnde centrale generatoren, leidend tot een ‘vertikale naar horizontale’ transformatie van het elektriciteitsvoorzieningsysteem. De nadruk ligt op het gebruik van de vermogenelektronisch gekoppelde DG als actief vermogen leverancier. Daardoor, anders dan Hoofdstuk 4, impliceert het toenemende DG niveau een vermindering van roterende massa (inertie) en regelmogelijkheden van blindvermorgen in het net. In sommige gevallen wordt instabiliteit van het transmissienet bij een hoog DG niveau (meer dan 50%) gevonden. Verschillende oplossingen voor dit probleem worden voorgesteld middels verandering van de inzet van de centrale generatoren en optimalisering van de vermogenstransporten. In sommige gevallen moet een minimaal aantal centrale generatoren in bedrijf gehouden worden om instabiliteit van het net te.

(11) Samenvatting. vii. vermijden. De resultaten van de Hoofdstukken 4 en 5 zijn gebaseerd op een deterministische benadering, waarbij de parameters van het testsysteem ingesteld zijn op hun typische waarden. Behalve een deterministische benadering kan ook een stochastische analyse worden gebruikt om de transiënte stabiliteit te bestuderen, zoals beschreven in Hoofdstuk 6. De nadruk van de studie ligt op de invloed van het stochastische gedrag van DG. De resultaten tonen aan dat het meenemen van het stochastische gedrag van DG leidt tot een vollediger en gedetailleerder overzicht van het functioneren van het systeem. Hoofdstuk 7 tot slot behandelt de situatie waarbij het transmissienet aan een extreem scenario onderwerpen wordt, waar DG de totale belasting van het net dekt (100% DG niveau). DG eenheden zijn via vermogenselektronische interfaces opgenomen in zogenaamde “actieve distributie systemen” (ADS) en aangenomen is dat deze ook met vermogenselektronische interfaces zijn gekoppeld met het transmissienet. Het systeem is verondersteld nog steeds verbonden te zijn met een bron met constante 50 Hz frequentie bedoeld als een frequentie referentie voor de generatoren, maar wekt geen vermogen op in de stationaire toestand. Daarom moet iedere onbalans in vermogen door opwekking in de ADS gecompenseerd worden. Echter, vanwege de vermogenselektronische interfaces is het uitgangsvermogen van de DG eenheden (en de ADS) ontkoppelt van de netfrequentie. Daarom wordt in dit hoofdstuk de spanning gebruikt om de vermogensbalans te detecteren en te handhaven. Voor dit doel zijn specifieke regelconcepten ontwikkeld. De simulatieresultaten tonen aan dat door toepassing van deze regeltechnieken, de handhaving van de vermogenbalans gerealiseerd kan worden via de ADS. Het onderzoek beschreven in dit proefschrift toont aan dat vanuit oogpunt van stabiliteit van het transmissienet voldoende inertie en spanningsondersteuning aanwezig moet zijn als hogere DG penetratieniveaus aan de orde zijn. Verder moet men zich bewust zijn van het feit dat het systeem zich stochastisch gedraagt, zeker met DG. In zekere mate kan regionale balanshandhaving worden uitgevoerd met lokale spanningsregeling..

(12) viii. Samenvatting.

(13) Contents Summary in English. i. Samenvatting in het Nederlands. v. Contents 1 Introduction 1.1 ’Vertical’ Power Systems . . . . . 1.2 Distributed Generation Concept 1.3 ’Horizontal’ Power Systems . . . 1.4 Dynamics of Power Systems . . . 1.5 Research Framework . . . . . . . 1.6 Objectives and Limitations . . . 1.7 Outline of the Thesis . . . . . . .. ix. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. 1 1 3 4 4 7 8 9. 2 Distributed Generation 2.1 State-of-the-art DG Technology . . . . . . . . . . . 2.1.1 Conventional Fossil-Fuel Based Generators 2.1.2 Microturbines . . . . . . . . . . . . . . . . . 2.1.3 Combined Heat and Power (CHP) Plants . 2.1.4 Small Hydro-Power Plants . . . . . . . . . . 2.1.5 Wind Turbines . . . . . . . . . . . . . . . . 2.1.6 Photovoltaics . . . . . . . . . . . . . . . . . 2.1.7 Fuel Cells . . . . . . . . . . . . . . . . . . . 2.1.8 Geothermal Power Plants . . . . . . . . . . 2.1.9 Biomass Power Plants . . . . . . . . . . . . 2.1.10 Tidal Power Plants . . . . . . . . . . . . . . 2.1.11 Wave Power Plants . . . . . . . . . . . . . . 2.2 Output Power Characteristics . . . . . . . . . . . . 2.2.1 Controllable DG . . . . . . . . . . . . . . . 2.2.2 Non-controllable DG . . . . . . . . . . . . . 2.3 Energy Storage Systems . . . . . . . . . . . . . . . 2.3.1 Batteries . . . . . . . . . . . . . . . . . . . 2.3.2 Hydrogen Fuel Cells . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. 11 11 12 12 12 13 13 13 13 14 14 14 15 15 16 16 19 19 19. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . ..

(14) x. Contents. 2.4. 2.5 2.6. 2.3.3 Redox Flow Batteries . . . . . . . . . . . . . . . . . . . . 2.3.4 Flywheel Systems . . . . . . . . . . . . . . . . . . . . . . . 2.3.5 Ultracapacitors . . . . . . . . . . . . . . . . . . . . . . . . 2.3.6 Superconducting Magnetic Energy Storage (SMES) Systems 2.3.7 Pumped-Hydroelectric Plants . . . . . . . . . . . . . . . . 2.3.8 Compressed-Air Systems . . . . . . . . . . . . . . . . . . . DG Grid-Connection Characteristics . . . . . . . . . . . . . . . . 2.4.1 Direct Grid-Connected DG . . . . . . . . . . . . . . . . . 2.4.2 Indirect Grid-Connected DG . . . . . . . . . . . . . . . . 2.4.3 Connecting Energy Storage to the Grid . . . . . . . . . . DG Prospects: Converter-Connected DG . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20 20 20 20 20 21 22 22 23 26 26 27. 3 Stability of Systems with DG 3.1 Classification of Power System Stability . . . . . . . . 3.1.1 Rotor Angle Stability . . . . . . . . . . . . . . 3.1.2 Voltage Stability . . . . . . . . . . . . . . . . . 3.1.3 Frequency Stability . . . . . . . . . . . . . . . . 3.2 Rotor Dynamics of Synchronous Machines . . . . . . . 3.2.1 Swing Equation . . . . . . . . . . . . . . . . . . 3.2.2 Power-Angle Equation . . . . . . . . . . . . . . 3.2.3 Equal Area Criterion . . . . . . . . . . . . . . . 3.3 System Stability Indicators . . . . . . . . . . . . . . . 3.3.1 Maximum Rotor Speed Deviation . . . . . . . . 3.3.2 Oscillation Duration . . . . . . . . . . . . . . . 3.4 DG and Large System Dynamic Simulation . . . . . . 3.4.1 Modeling DG Technologies . . . . . . . . . . . 3.4.2 Power System Dynamics Software Packages . . 3.5 Simulation Setup . . . . . . . . . . . . . . . . . . . . . 3.5.1 The IEEE 39-bus New England Test System . 3.5.2 DG Technology . . . . . . . . . . . . . . . . . . 3.5.3 Incorporation of DG in Distribution Networks . 3.5.4 Behavior of Centralized Power Plants . . . . . 3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. 29 29 30 31 31 31 31 33 34 36 37 37 38 39 41 43 43 44 46 47 47. 4 Impact of DG on Power System Transient Stability 4.1 DG Impacts . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Simulation Scenarios . . . . . . . . . . . . . . . 4.1.2 Transient Stability Simulation . . . . . . . . . . 4.1.3 Simulation Results . . . . . . . . . . . . . . . . 4.1.4 Remarks . . . . . . . . . . . . . . . . . . . . . . 4.2 DG Grid-Connection Strength Impacts . . . . . . . . . 4.2.1 Distribution Network and DG Layout . . . . . 4.2.2 Simulation Scenarios . . . . . . . . . . . . . . . 4.2.3 Transient Stability Simulation . . . . . . . . . . 4.2.4 Simulation Results . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. 49 49 49 50 51 54 56 56 57 60 60.

(15) Contents . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. 62 63 63 64 64 69 69 69 71 71 72 72. 5 ’Vertical to Horizontal’ Transformation of Power Systems 5.1 Simulation Setup . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Simulation Results Case I . . . . . . . . . . . . . . . . . . . . 5.3 Rescheduling Generation Case I . . . . . . . . . . . . . . . . . 5.4 Simulation Results Case II . . . . . . . . . . . . . . . . . . . . 5.5 DG with Ride-Through Capability . . . . . . . . . . . . . . . 5.6 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . .. 75 76 78 83 84 88 89 89. 4.3. 4.4. 4.5. 4.2.5 Remarks . . . . . . . . . . . . . . . . . . DG Penetration Level and Technology Impacts 4.3.1 Simulation Scenario . . . . . . . . . . . 4.3.2 Transient Stability Simulation . . . . . . 4.3.3 Simulation Results . . . . . . . . . . . . 4.3.4 Remarks . . . . . . . . . . . . . . . . . . Protection of Power-Electronics Impacts . . . . 4.4.1 Simulation Scenarios . . . . . . . . . . . 4.4.2 Transient Stability Simulation . . . . . . 4.4.3 Simulation Results . . . . . . . . . . . . 4.4.4 Remarks . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . .. xi . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. 6 Stochastic Approach to Transient Stability of Power Systems with DG 91 6.1 Stochastic Load Flow . . . . . . . . . . . . . . . . . . . . . . . . 92 6.2 Stochastic Transient Stability Analysis . . . . . . . . . . . . . . . 92 6.2.1 Simulation Scenario . . . . . . . . . . . . . . . . . . . . . 92 6.2.2 Monte Carlo simulation (MCS) Samples . . . . . . . . . . 94 6.2.3 Simulation Results . . . . . . . . . . . . . . . . . . . . . . 95 6.3 Stochastic Transient Stability Study with Increasing DG . . . . . 99 6.3.1 Simulation Scenario . . . . . . . . . . . . . . . . . . . . . 99 6.3.2 MCS Samples . . . . . . . . . . . . . . . . . . . . . . . . . 99 6.3.3 Simulation Results . . . . . . . . . . . . . . . . . . . . . . 100 6.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 7 Maintaining Power Balance with Active Distribution Systems 105 7.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 7.2 Power Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 7.3 Model of Power System with ADS . . . . . . . . . . . . . . . . . 107 7.3.1 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 107 7.3.2 Model of ADS . . . . . . . . . . . . . . . . . . . . . . . . 107 7.3.3 Generator Models . . . . . . . . . . . . . . . . . . . . . . 108 7.4 Basic Controller Model . . . . . . . . . . . . . . . . . . . . . . . . 110 7.5 ADS Control Systems . . . . . . . . . . . . . . . . . . . . . . . . 113 7.5.1 Stand-alone master controller . . . . . . . . . . . . . . . . 114 7.5.2 Decentralized-controller with single reference . . . . . . . 116.

(16) xii. Contents. 7.6. 7.5.3 Decentralized controller with hysteresis . . . . . . . . . . 118 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120. 8 Conclusions 8.1 Overview . . . . . . . . . . . . 8.2 Stochastic Stability Studies . . 8.3 Remarks and Future Works . . 8.3.1 ‘Inertia’ Contribution . 8.3.2 Reactive Power Control. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. 123 123 125 125 125 126. A List of Symbols and Abbreviations. 127. B Test System Data. 131. C Generator, Governor and Excitation Systems Data. 135. D Power Flow Computation 139 D.1 Power Flow Problem . . . . . . . . . . . . . . . . . . . . . . . . . 139 D.2 Newton-Rhapson power flow solution . . . . . . . . . . . . . . . . 141 Bibliography. 143. Scientific Contributions. 151. Acknowledgment. 155. Biography. 157.

(17) Chapter 1. Introduction A classical power system is characterized by a relatively small number of large, so-called centralized power plants for meeting the electric energy demand. These power plants are built based on the demand estimate for a certain period of time. Some constraints, however, limit the expansion and use of such large power plants and may induce a shift towards a more extensive use of small, decentralized power generators. It is well-known that the implementation of small, decentralized power generators brings both positive and negative consequences to the existing power system. The technical consequences must be considered carefully in order to maintain the present reliability level of the power system. Some of these negative consequences will be focused on in this work. Several remedies to eliminate them or limit their impact will be suggested and discussed.. 1.1. ’Vertical’ Power Systems. A power system is designed to supply electrical power to the consumers. Until now, mainly large, centralized generators have been utilized for the power generation. Synchronous generators are typical electromechanical energy transducers for large power plants, often close to cooling water, energy resources or supply routes and connected to the transmission system. If hydropower is available it may be used as input too. So, a classical power system consists of three technical stages, namely: generation, transmission, and distribution. The generation system converts mechanical power that results from the conversion of primary energy sources, such as nuclear, hydro power, coal, gas, etc., into electrical. The transmission system transports the electrical power over a long distance to the load centers. The distribution system distributes the electrical power to the consumers/loads. This classical power system can be best illustrated by considering the different voltage levels (Figure 1.1)..

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(22) Introduction After being generated at the power plant (typically at a voltage of 10 kV to 30 kV), the power is transformed to a higher voltage level in the generation substation. The High Voltage (HV) or Extra High Voltage (EHV) (110 kV to 400 kV) transmission systems transports the electrical power further to the (sub)transmission substations, where the power is transformed to Medium Voltage (MV) level (typically 10 to 30 kV), and where it enters the primary distribution systems. Finally, in the distribution substations, the power is transformed to Low Voltage (LV) level and is distributed to the consumers [78]. Therefore, electrical energy generally flows from the higher to the lower voltage levels in the network. Based on the different voltage levels, this type of power system can then be viewed as a ’vertically-operated’ power system, which in this work will be referred to as a ’vertical’ power system. The expansion and the construction of large power plants are limited by both rational (e.g. economical, environmental and geographical) considerations and irrational constraints (e.g. social and political issues) [56].. 1.2. Distributed Generation Concept. Nowadays, an increasing amount of electrical power is generated by decentralized power generators of relatively small scale (i.e. smaller than 50-100 MW). This way of electrical power generation is referred to as ’Distributed Generation’ (DG) because it is spread out over the system. These small power generators are usually located in the vicinity of the electrical loads, and are mostly connected to distribution networks (i.e. at MV- or LV-networks) [12], [32]. In contrast to the conventional power plants, the development and the implementation of DG units are encouraged mostly by environmental forces. This has stimulated research, promotion, development and increased use of new, renewable, clean and environmentally friendly forms of energy [5], [27]. Renewable energy sources like wind, biomass, sun, tidal-, wave- and geothermal energy are used. Most of these renewable energy sources can be converted to electric power, in units in a range of hundreds kWs to some MWs, by (relatively) small generators that are connected to the distribution networks, close to the load centers. Some types of distributed generation (DG) are based on conventional fossil energy sources, but are often because of their relatively low carbon emission classified as environmentally-friendly types of power generation. Within this class are the microturbine generator supplied by natural gas, and the Combined Heat and Power (CHP) generation, which is practically a parallel conversion of fuel into electrical and thermal energy (more carbon emission will be produced if the electrical and thermal energy are generated separately). The rise of DG is supported by the advancements in supporting technologies like power electronic converters and controllers. Currently there are many DG technologies available. An overview of these DG technologies is given in Chapter 2.. 3.

(23) 4. 1.3 ’Horizontal’ Power Systems. 1.3. ’Horizontal’ Power Systems. Due to the possible large-scale implementation of DG units in the classical ’vertical’ power system, a transition towards a more ’horizontal’ power system may take place. In addition to the power injected in the EHV and HV system by the large power plants, DG units supply the system via the MV or LV networks. Therefore, the power can flow both ’vertically’, i.e. from the higher to the lower voltage levels, as well as ’horizontally’ from one MV or LV network to another or from a generator to a load within the same MV or LV network leading to a new term: the horizontal power system or horizontally-operated power system. The implementation of DG turns the passive distribution network into an active one. In this active distribution network some costumers not only consume electricity, but they also generate and if generation surpasses their demand, supply the network. Figure 1.2 shows an example of both an active and a passive distribution network. In the active network the power flow is no longer in one direction (downwards), as is the case in the passive network, but may be bidirectional (down- and upwards). In this way, it is possible that power is transferred from one distribution network to another. When we reflect further on this issue, we could even imagine that on certain moments in time the electrical power generated by the DG within the distribution networks may become sufficient to fulfill the total demand of the system. In this particular case, the remaining large (centralized) power plants may be shut down. Figure 1.3 illustrates a ’Vertical-to-Horizontal’ transformation of a power system. In the ’first’ transformation step, a large amount of DG is implemented in the power system and all centralized generators remain but generate less (Figure 1.3: graph (a) changes to graph (b)). In the ’second’ transformation step, the amount of DG in the system increases in such a way that a number of centralized generators (power plants) are shut down for efficiency reasons (Figure 1.3: graph (b) changes to graph (c)). Finally, in the ’third’ transformation step, when power generated by DG within the active distribution networks is sufficient to match the total demand, all (remaining) centralized generators are out of service (Figure 1.3: graph (c) to graph (d)). It is hard to imagine how a system under graph d could operate but theoretically these are the steps.. 1.4. Dynamics of Power Systems. The power system is a dynamic system. Even under normal operation conditions, loads are connected and disconnected frequently and demand for both active and reactive power changes continuously. Besides, the power system is subject to disturbances caused by malfunctioning or failing equipment. The ability of a power system to remain in a state of operating equilibrium under normal operating conditions and to regain an acceptable state of equilibrium after being subjected to a disturbance is defined as the power system stability [34]..

(24) Introduction. 5. Transmission Network. Industrial customers who consume electricity. Distribution Network. Domestic customers and small business who consume electricity. Thin line indicates flow from the network. (a) Passive Network. Transmission Network Industrial customers with DG (colored in gray) also generate some electricity which flows back into the network. Distribution Network. Distributed generator e.g. wind turbine. Domestic customers and small business with domestic DG (colored in gray) can also generate electricity which flows back into the network Thin line indicates flow from the network Thicker line indicates flow from, and to, the network. (b) Active Network. Figure 1.2: Conventional (passive) distribution network (top - a) and an active distribution networks with DG (bottom - b) (modified from [14]).

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(29). Figure 1.3: ’Vertical-to-Horizontal’ transformation of the power system. More than 100 years of experience have lead to the present power system with many subsystems and dynamic elements, a lot of them equipped with control systems to stabilize the operation of the overall system. For instance, power plants are equipped with prime mover controllers to regulate the speed and thus the active power output, and excitation controllers to regulate the voltage and thus the reactive power output. When we consider the interconnected power system as a single system, a necessary condition for a stable operation is that more or less all the large synchronous generators remain in synchronism. Then it is up to the dynamics of the system whether stability can be maintained. Instability can also occur if a power system cannot maintain the voltage levels within a required range [34]. The implementation of DG influences both steady state and dynamic performance of a power system. In this work, however, the focus is on the dynamics..

(30) Introduction. 1.5. Research Framework. The research presented in this work has been performed within the framework of the ’Intelligent Power Systems’ project. The project is part of the IOP-EMVT program (Innovation Oriented research Program - Electro-Magnetic Power Technology), financially supported by SenterNovem, an agency of the Dutch Ministry of Economical Affairs. The ’Intelligent Power Systems’ project is initiated by the Electrical Power Systems and Electrical Power Electronics Groups of the Delft University of Technology and the Electrical Power Systems and Control Systems Groups of the Eindhoven University of Technology. In total 10 Ph.D. students are involved and work closely together. The research focuses on the effects of the structural changes in generation and demand taking place, like for instance the large-scale introduction of distributed (renewable) generators [59]. The project consists of four parts (illustrated in Figure 1.4). The first part (research part 1), inherently stable transmission system, investigates the influence of uncontrolled decentralized generation on stability and dynamic behavior of the transmission network. As a consequence of the transition in the generation, less centralized plants will be connected to the transmission network as more generation takes place in the distribution networks, whereas the remainder is possibly generated further away in neighboring systems. Solutions investigated include the control of centralized and decentralized power, the application of power electronic interfaces and monitoring of the system stability. The second part (research part 2), manageable distribution networks, focuses on the distribution network, which becomes ’active’. Technologies and strategies have to be developed that can operate the distribution network in different modes and support the operation and robustness of the network. The project investigates how the power electronic interfaces of decentralized generators or between network parts can be used to support the grid. Also the stability of the distribution network and the effect of the stochastic behavior of decentralized generators on the voltage level are investigated. In the third part (research part 3), self-controlling autonomous networks, autonomous networks are considered. When the amount of power generated in a part of the distribution network is sufficient to supply a local demand, the network can be operated autonomously but as a matter of fact remains connected to the rest of the grid for security reasons. The project investigates the control functions needed to operate the autonomous networks in an optimal and secure way. The interaction between the grid and the connected appliances has a large influence on the power quality. The fourth part (research part 4), optimal power quality, of the project analyzes all aspects of power quality. The goal is to provide elements for the discussion between polluter and grid operator who has to take measures to comply with the standards and grid codes. Setting up a power quality test lab is an integral part of the project. The research described in this thesis is within research part 1: inherently stable transmission systems.. 7.

(31) 8. 1.6 Objectives and Limitations. 1 Inherently stable transmission system. 4. 2. 3. Optimal power quality. Manageable distribution networks. Self-controlling autonomous networks. Figure 1.4: Research items within the ’Intelligent Power Systems’ research project. 1.6. Objectives and Limitations. In this work, the following two objectives are set: • Investigate the impact of a high DG penetration level on the stability of a power system. • Investigate the stability of a power system that undergoes a ’vertical-tohorizontal’ transformation. This research is unique as it combines the investigation of an increasing DG penetration level and a power system that transforms from a vertical into a horizontal one. The focus is only on the dynamic impacts, i.e. the transient stability. Steady state and economic impacts are beyond the scope of this work. For this purpose, power system simulation software package PSS/E is used, where models of the power (test) system and DG are included. Simulation scenarios of power systems with DG are defined later. Based on the simulation results, special emphasis is on the behavior of the centralized generators in service..

(32) Introduction. 1.7. Outline of the Thesis. The thesis is organized as follows: • In Chapter 2, an overview of the current DG technologies is given. The emphasis is on the classification of the different DG technologies according to their potential impact on the power system stability. • An overview of power system stability is presented in Chapter 3. In this chapter, the term ”inherently stable transmission system” is defined. Furthermore the research approach and the simulation setup, used and presented throughout the work, are discussed, including test system, software, system stability indicators, basic associated controls of system elements and parameters used. • In Chapter 4, the impact of DG implementation on the power system transient stability is discussed. Different scenarios of a power system with a high DG penetration level are developed. The impact of different DG technologies, fault durations and locations, DG penetration level, DG gridconnection-option, and protection schemes of power electronic interfaced DG units are investigated. • In Chapter 5, the scenarios for the ’vertical-to-horizontal’ transformation of power systems are further elaborated. The transient stability impact of this transformation is analyzed and solutions for reducing the negative effects are discussed. • In Chapter 6, a stochastic approach to the transient stability of system with DG within the framework of “vertical-to-horizontal” transformation is studied. • In Chapter 7, a power system reaching 100% DG implementation is studied. DG units are implemented within Active Distribution Systems. Control methods to maintain the power balance in such a power system with Active Distribution Systems are suggested. • The conclusions and recommendations for future work are given in Chapter 8.. 9.

(33) 10. 1.7 Outline of the Thesis.

(34) Chapter 2. Distributed Generation The impact of distributed generation (DG) on the dynamic stability of power systems is studied. Therefore, it is important to examine characteristics of DG that influence this behavior. As mentioned in Chapter 1, DG units can be based on various (both conventional and alternative) primary energy sources. The type of primary energy source and the conversion process determine, to a large extent, the output power characteristics of DG and the type of grid connection applied. Based on the output power characteristics, DG can be classified as dispatchable or non-dispatchable as is described in Section 2.2. The output power of non-dispatchable units, especially the ones driven by renewable energy sources, can show high output-power fluctuations. Energy storage systems, as described in Section 2.3, can be applied to smooth this intermittent effect. In Section 2.4 the way DG is connected to the network (grid) is reviewed. There are two options, depending on both the type of primary energy source and prime mover: a direct and indirect grid connection. A direct grid connection is made by using the common/classical synchronous and induction generators, whereas an indirect grid connection is made by means of power-electronic converters. Section 2.5 elaborates on this issue. Concluding remarks are made in Section 2.6.. 2.1. State-of-the-art DG Technology. Many definitions of distributed generation (DG) exist. CIGRE Working Group 37.23, for example, has defined distributed generation (DG) as electrical generation that is not centrally planned, not centrally dispatched∗ , and connected to the distribution network [12]. A DG unit usually produces electric power well below 100 MW [32], [33]. Other literature however advocates a boarder and ∗ not centrally dispatched: it cannot be controlled from a system control center; it does not implicate that the unit cannot be controlled locally.

(35) 12. 2.1 State-of-the-art DG Technology. more straightforward definition of DG: a DG source is an electric power generation source connected directly to the distribution network or on the customer side of the meter [1], [52]. Thus, the way that a generator is implemented in a power system determines its classification as DG, and not the type of primary energy source used. However, many generator units that are driven by renewable sources of energy inherently possess the characteristics of DG.. 2.1.1. Conventional Fossil-Fuel Based Generators. Within the category DG, the term ’Conventional Fossil-Fuel Based Generator’ is used to describe small fossil-fueled power plants within a range of kWs up to 100 MW [7], [12], [33]. The reciprocating engines and gas turbines are the most common in this category. Reciprocating engines are characterized by low capital cost, possible thermal and electrical cogeneration, and good modularity and flexibility. Furthermore they are reliable [7], [12]. Reciprocating engines, however, have drawbacks. The use of diesel or gasoline gives high emission levels [30]. The emission can be reduced to some extent by using natural gas as energy source. A large number of moving parts leads to high noise levels pollution and increases the maintenance cost [7]. Gas turbines are commonly used in industry [12]. In oil industry for example, the associated gas from the oilfield is frequently used to generate electricity. The use of natural gas results in lower emission when compared to reciprocating engines. As DG, gas turbines are mostly encouraged by the development of microturbines, highlighted in the next subsection.. 2.1.2. Microturbines. A ’micro’ gas turbine (microturbine) produces electric power in the range of 25500 kW. An electrical generator is integrated within the microturbine, that operates at a high speed (50,000 to 120,000 RPM). The electric power is produced with a frequency (in the order) of thousands of Hz [2]. Therefore, a powerelectronic converter is used to interface the generator and the grid. Within the power-electronic interface, the high-frequency electrical power is converted to DC before it is inverted back to the low-frequency AC of the grid. Most microturbines use natural gas. As a consequence, microturbines are typically characterized by low emission levels. The use of renewable energy sources such as ethanol is also possible [15].. 2.1.3. Combined Heat and Power (CHP) Plants. Combined Heat and Power (CHP), also known as cogeneration, is the simultaneous production of electrical power and useful heat [33]. Reciprocating engines, gas turbines, and microturbines can be used in CHP schemes. CHP generation on a large scale is usually based on fossil fuel. In general, CHP is heat driven.

(36) Distributed Generation. 13. and electricity is the by-product. With this simultaneous process, the overall efficiency of a CHP plant can be around 85% [12], [33].. 2.1.4. Small Hydro-Power Plants. A hydro-power plant generates electricity from the motion of a mass of water, where a power house is installed. This water movement can be obtained, for example, from a run-of river or a river with a small impoundment [66]. A small hydro-power plant produces electric power up to 10 MW. Hydro power plant technology has reached maturity. A small hydro-power plant has less impact on the environment and ecosystem, when compared to a large hydro-power plant, and is easy to build within a short construction schedule [66]. Once built, its maintenance cost is minimal [62].. 2.1.5. Wind Turbines. A wind turbine generates electricity by extracting kinetic energy from the wind passing through its blades. Wind energy is one of the most promising energy sources to be used for renewable electricity generation [51]. Apart from using a small wind turbine as DG for generating emission-free power, however, the increasing interest for implementing wind turbines is mostly driven by the availability of wind energy for generating power at large scale of MWs or even GWs [77].. 2.1.6. Photovoltaics. Photovoltaic (PV) power generation systems convert sunlight directly into electricity [51]. A PV cell consists of two or more semiconductor layers of specific physical properties. These layers are arranged in such a way that when the PV cell is exposed to sunlight, the photons cause the electrons to move in one direction (crossing the junctions of the layers) and a direct current (DC) is generated. Currently, PV energy cost is still high. However, the capital cost of PV modules has declined in the past decades. PV implementation is encouraged by the infinite availability of sun energy, long life cycle and simple maintenance (since there are no moving parts), high modularity and mobility, and short design, installation and start-up time of a new plant [51].. 2.1.7. Fuel Cells. DC power can also be generated by an electrochemical process. An example is the so-called fuel cell. It consists of a positive electrode (anode) and a negative electrode (cathode). To generate electricity, fuel (usually hydrogen) and an oxidant must be supplied to the anode and the cathode, respectively. Electrochemical reactions create ion flows, that generate electricity. One fuel cell.

(37) 14. 2.1 State-of-the-art DG Technology. only produces a small amount of electricity, and larger amounts can be obtained from a stack of fuel cells [7], [13]. Fuel cells are modular, portable and produce low noise pollution, because there are no moving parts [39]. These characteristics make fuel cells suitable as DG in, for examples, remote areas. In the future, electrical networks (both AC and DC) can be combined with a gas and hydrogen infrastructure. This new structure may further increase the implementation of fuel cells as DG [19].. 2.1.8. Geothermal Power Plants. Geothermal power plants convert the energy contained in hot rock into electricity by using water to absorb the heat from the rock and transport it to the surface of the earth. The heat from geothermal reservoirs provides the force that rotates the turbine generators and produces the electricity. The used geothermal water is then returned (injected back) into the reservoir to be reheated. This cycle will maintain the pressure of the reservoir and sustain the reservoir [57]. A geothermal power plant is relatively sustainable.A field may remain productive over a period of tens of years. It produces no pollutant and no unwanted product, if any, can be disposed underground [68], [80].. 2.1.9. Biomass Power Plants. The term “biomass” describes all organic matter that is produced by photosynthesis. It includes all water- and land-based vegetation and trees, municipal biosolids (sewage), animal wastes (manures), forestry and agricultural residues, and certain types of industrial wastes [26], [72]. Biomass is considered a substitute for fossil fuels. Practically, biomass is converted to thermal energy, liquid, solid or gaseous fuels and other chemical products through a variety of conversion processes [72]. The latter forms are then converted into electricity. The biomass products, for example, can be used as fuel to generate electricity. The gaseous fuels can be applied in fuel cell systems. In general, biomass is abundantly available and can be considered as a renewable.. 2.1.10. Tidal Power Plants. Tidal energy is derived from the gravitational forces of attraction that operate between the earth and the moon, and the earth and the sun. Energy is extracted either directly by harnessing the kinetic energy of currents due the tides or by using a basin to capture potential energy from the difference in height of a rising and falling mass of water. To generate electricity, tidal flow is extracted by means of propellers with large diameters. In the latter technique, a huge dam, called a ’barrage’ is built across a river estuary. When the tide goes in and out, the water flows through tunnels in the dam. The ebb and flow of the tides can be used to turn a turbine. When the tides comes into the shore, they.

(38) Distributed Generation. 15. can be trapped in reservoirs behind dams. Later, when the tide drops, the water dam can be used like in a regular operation of a hydroelectric power plant [16]. Tidal power is a renewable energy source. Tidal power plants produce no pollutant. They also cause no fundamental change of the natural rhythm of the tidal cycle and no inundation of the adjacent area. These factors encourage the implementation of tidal power plants [6]. However, building a tidal power plant has to be planned carefully considering the potential ecological impacts, especially during the construction [22].. 2.1.11. Wave Power Plants. Waves are generated at the surface of oceans by wind effects which in turn result from the differential heating of the earth’s surface. Wave energy is complementary to tidal power, it uses the essentially up-and-down motion of the sea surface (wave power), instead of using the energy of the sea rushing backwards and forwards (tidal power). A wave power plant extracts wave energy and converts it into electricity [79]. The wave power plant is promoted as electricity generation available in abundance throughout the world; it is clean and non-polluting, renewable, and suited to electrify remote communities, especially as DG [17]. However, just like a tidal plant, the erection of a wave power plant should be planned carefully, so that the ecological impacts are minimized.. 2.2. Output Power Characteristics. One of the main characteristics of a power system is that the supply and the demand must be kept in balance at any time. In a steady-state operation of a traditional power system, the use of synchronous generators (within the power plants) enables the power output of each plant (and each generating unit within the plant) to be dispatched for any specified load condition [25]. Dispatching a power plant (and a generator unit) is a function of the availability of the primary energy sources that drives the prime mover and the flexibility of the conversion process (ramping up and down). By definition DG units are not centrally dispatched† , but several DG technologies enable the DG unit to be controlled locally. The DG operator can determine an exact power output of the DG units by controlling the primary energy sources (or fuels) that are supplied to the DG units. Other DG technologies are based on renewable energy sources where the operator cannot dispatch the DG units because the behavior of the primary energy sources cannot be controlled. In case of wind turbines and photovoltaic panels, for example, no extra primary energy can be supplied to the generator units in order to produce more electricity. Normally, most of the renewable energy based electrical generation is operated in such a way that the electricity production is maximized. † the non-dispatchable characteristics of DG units are important when a stochastic approach is considered. Such approach is applied in this thesis in Chapter 6.

(39) 16. 2.2 Output Power Characteristics. Table 2.1: Controllable and Non-controllable Classification of DG DG technology Conventional Fossil-Fuel Based Generators Microturbines Combined Heat and Power (CHP) Plants Small hydro-power plants Wind turbines Photovoltaics Fuel cells Geothermal power plants Biomass power plants Tidal power plants Wave power plants. Controllable √ √. Non-controllable. √ √ √ √ √ √ √ √ √. In short, DG technologies can be classified into two categories: • Controllable DG. • Non-Controllable DG.. As a summary, Table 2.1 lists the various DG technologies and their classification as controllable and/or non-controllable generation.. 2.2.1. Controllable DG. Controllable DG is characterized by its ability to control the fuel supply to the generator. As a result, the output power can be determined, and dispatched. Among the DG technologies that can be classified as controllable DG are conventional fossil-fuel based generators, microturbines, fuel cells, geothermal power plants, and power plants driven by biomass. Except for the fuel cells, these technologies utilize conventional rotating electrical machines for power conversion (synchronous or induction generators), driven by prime movers based on reciprocating or combustion turbine technologies. By controlling the fuel that is supplied to the prime mover, the torque of the prime mover can be adjusted. A note should be made with regard to geothermal power plants. The geothermal primary energy source is not as flexible as fossil fuels for dispatching the generator units [68].. 2.2.2. Non-controllable DG. Non-controllable DG represents DG technologies where the DG operator cannot determine the power output of the DG units. Among the DG technologies that can be classified as non-controllable DG are small hydro power plants, wind turbines, photovoltaics, tidal- and wave power plants and CHP plants..

(40) Distributed Generation. 17. Small hydro-power plants Because of the non-availability of large power impounding (dam), the power output of a hydro turbine (the prime mover in the small hydro-power plant) is practically driven by a direct-captured water flow. A simple expression of the power output for a small hydro-plant is [33] P = QHηρg,. (2.1). with P the output power [W], Q the flow rate [m3 s−1 ], H the effective head [m], η the overall efficiency, ρ the density of water [kgm−3 ], and g the gravitional constant [ms−2 ]. For small hydro-power plants, H, η, ρ, and g in (2.1) are deterministic and constant. Without significant storage capacity, a small hydro-power plant may experience a very large variation in available water flow (Q), and output power (P ) [33]. Thus, a small hydro-power unit is non-dispatchable. Wind turbines The power generated by a wind turbine (provided that the upstream wind velocity, v, is between minimal and maximal values, e.g. 4 < v < 25 [ms−1 ]) can be expressed as [33], [51] P =. (1 + 1 Cp ρv 3 A, with Cp = 2. vo v )[1. 2. − ( vvo )2 ]. .. (2.2). In (2.2), P denotes the output power [W], Cp the power coefficient, vo the downstream wind velocity at the exit of the rotor blades [ms−1 ], ρ the air density [kgm−3 ], and A the swept area of the rotor blades [m2 ]. In practice, ρ, A, and to some extent Cp , are deterministic and constant values. Thus, the power produced by a wind turbine is mainly characterized by the wind velocity. The wind velocity itself has a stochastic nature; any wind speed can occur at any time [61]. Moreover, when the upstream wind velocity (v) is either below minimal or above maximal operating values of the wind plant, e.g. v < 4 or v > 25 [ms−1 ], the output power equals zero. As a result, a stochastic output power results, especially when a single wind turbine or plant is regarded [50]. Photovoltaics The power generated by a PV module is given in (2.3) [51] as. where. P = η × (Eed × AP V total + Ees × AreaP V withsun ),. (2.3). − → − → AP V withsun = ( S × P ) × AP V total. (2.4).

(41) 18. 2.2 Output Power Characteristics. and − → S − → P − → Sx − → Sy − → Sz − → Px − → Py − → Pz. − → = [Sx Sy Sz ], | S | = 1, − → = [Px Py Pz ], | P | = 1, = cos(θ) × cos(αsun ),. (2.5) (2.6) (2.7). = cos(θ) × sin(αsun ),. (2.8). = sin(θ),. (2.9). = cos(β) × cos(αpanel ),. (2.10). = cos(β) × sin(αpanel ),. (2.11). = sin(β).. (2.12). In (2.3) to (2.12) P denotes the power extracted from the sunlight [W], η the efficiency of the solar panel, Eed and Ees the diffuse- and the direct-horizontal − → − → irradiance [Wm−2 ], S and P the solar- and panel orientation, θ and αsun the altitude- and azimuth angle of the sun [rad], and β and αpanel the altitude- and the azimuth angle of the panel [rad]. In practice, AP V total , η, β and αpanel are deterministic and constant. Therefore, the generated electricity is characterized by Eed , Ees , θ and αsun . The altitude- and the azimuth angle of the sun (θ and αsun ) have daily and seasonal patterns, whereas the characteristics of Eed and Ees are intermittent. Weather changes and cloud movement, for example, strongly influence the values of Eed and Ees , and the generated electricity. The power generation of PV is noncontrollable [81]. Tidal power plants The power output of a turbine operating in flowing water is [74] P =. 1 ρACp v 3 . 2. (2.13). In (2.13), P denotes the output power [W], ρ the density of the fluid [kgm−3 ], A the area of the flow covered by the device [m2 ], Cp the power coefficient of the device (the percentage of power that the turbine can extract from the water flowing through the turbine), and v the velocity of the water [ms−1 ]. For a tidal power plant, ρ, A, and Cp in (2.13) are deterministic and constant. Therefore, the output power P depends on the velocity of the water v. Thus, the tide, which is predicable but variable in nature [9], is the only factor that affects the generating activity of a tidal power plant. This makes the tidal power generation non-controllable. Wave power plants The power production of a wave power plant can be assessed using [45], namely Pabs = αAw Hs1.5 .. (2.14).

(42) Distributed Generation. 19. In (2.14) Pabs denotes the average absorbed power, Aw the float water plain area [m2 ], and Hs the significant wave height [m].α is a coefficient that equals 0.166 [kgm−1.5 s−3 ] under ideal conditions [45]. For a wave power plant, α and Aw are deterministic. The output power depends practically on the wave height (Hs ), neither constant nor controllable. Hence, a wave power plant is non-controllable.. 2.3. Energy Storage Systems. A large-scale implementation of renewable energy sources for power generation could be expected in future. With the non-controllable nature of these generators, especially when renewable energy sources such as wind, wave and sun are exploited, the output power is difficult to predict. Moreover, high power fluctuations from these generators can be expected. In this case, energy storage systems/devices may be needed to cover the resulting imbalances between the power generation and the consumptions [64], [67]. In the following sections, energy storage systems, and their technical characteristics from a power system point of view, are highlighted.. 2.3.1. Batteries. Batteries store energy in electrochemical form. There are two basic types of batteries. The so-called primary battery converts chemical energy into electrical energy in a non-reversible process and is discarded after discharge. The secondary battery works in a reversible reaction. It converts chemical energy into electrical in discharge, and vice versa in charge mode [51]. Batteries are the most widely-used devices for electrical energy storage in a variety of applications. Batteries can store rather large amounts of energy in a relatively small volume. They are modular, so that output power in the order of MWs is accessible. They are also quiet during operation, which makes them suited for implementation near load centers where also DG can be implemented [36], [64].. 2.3.2. Hydrogen Fuel Cells. Hydrogen fuel cells store energy in electrochemical form. Hydrogen is produced by electrolysis of water using the off-peak electricity such as coming from wind turbines, photovoltaics, hydro or even nuclear power plants. This hydrogen can be used to operate fuel cells when there is a high demand for electricity. In this way, the hydrogen fuel cell energy storage can be ’charged’ and ’discharged’ reversibly. Hydrogen fuel cells are environmentally friendly, if the hydrogen is produced by electrolysis of water, and no fossil-based fuel is used [64]..

(43) 20. 2.3.3. 2.3 Energy Storage Systems. Redox Flow Batteries. Redox flow batteries store energy in electrochemical form. They are classified in between secondary batteries and hydrogen fuel cells and they have the characteristic of secondary batteries as they can be charged and discharged [54]. Flow batteries have a number of advantages. For example, the power output can easily be varied by increasing the size of the membranes, and the storage capacity can be raised by increasing the size of the tanks of the electrolytes [54].. 2.3.4. Flywheel Systems. Flywheels store energy mechanically as kinetic energy [64]. A flywheel system consists of a flywheel, a motor/generator, and power-electronic converter. The flywheel speeds up as it accumulates energy and slows down as energy is released [29]. A flywheel system has a high energy-storage density. It also has an unlimited number of charging and discharging cycles.. 2.3.5. Ultracapacitors. Ultracapacitors store energy in the form of electrostatic energy (static charge). Similar to a regular capacitor, the electric energy is stored by means of charge separation [10]. However, compared to ordinary electrolyte capacitors, ultracapacitor’s capacitance can be more than tens times higher. Its equivalent internal resistance is more than tens times lower than that of a battery, allowing more than tens times higher discharging/charging currents [23]. This makes the ultracapacitor suited for short-term, high-power applications. An ultracapacitor has an unlimited number of charge and discharge cycles at high rates [10], [23].. 2.3.6. Superconducting Magnetic Energy Storage (SMES) Systems. Superconducting Magnetic Energy Storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil. The stored energy can be released by discharging the coil. The use of SMES is encouraged by several factors [40]. Firstly, its efficiency is very high (up to 95%) as no conversion of energy to other forms is involved. Secondly, it can very rapidly dump or absorb power from the grid as the only limitations are the control loop and the switching time of the solid-state components connecting the coil to the grid.. 2.3.7. Pumped-Hydroelectric Plants. Pumped-hydroelectric plants store energy in the form of potential energy. They use off-peak power to pump water uphill to an elevated reservoir. When electricity is needed, the water is released to flow to a lower reservoir, and its potential energy is used to drive turbines [64]..

(44) Distributed Generation. 21. A pumped-hydroelectric energy system enables large-scale energy storage with a high capacity and power rating. It has an unlimited charging and discharging cycle and long life duration. The implementation of such an energy storage system is limited by the requirement of a significant land area with suitable topography for the upper and lower plants.. 2.3.8. Compressed-Air Systems. Compressed-air energy storage systems store energy in the form of potential energy by compressing air within an air reservoir, using a compressor powered by off-peak electric energy. When electricity is needed, the air is withdrawn, heated via combustion, and run through expansion turbines to drive an electric generator. During discharge, the plant’s generator produces power using a conventional natural gas combustor and the compressed air. When charging, the generator operates in reverse - as a motor (powered by the off-peak electric energ) - to provide mechanical energy to the air compressors [64], [65]. Such a plant uses about one-third of the premium fuel of a conventional simplecycle combustion turbine and produces one-third of the pollutants per kWh generated. This technology is considered as a hybrid storage and generation plant because it uses fuel and electricity in its storage cycle [64]. Among the positive aspects of a compressed-air energy storage system, are its availability for large-scale storage (high capacity and power rating), the unlimited charge and discharge cycles, and the long life duration [65], however it needs specific geography. Figure 2.1 shows an indication of the working areas of these energy storage systems. . 3XPSHG +\GURHOHFWULF 3ODQWV &RPSUHVVHG $LU6\VWHPV. )ORZ%DWWHULHV. &DSDFLW\>:K@. . +\GURJHQ)XHO&HOOV. . %DWWHULHV. . 60(6 )O\ZKHHOV. . 8OWUDFDSDFLWRUV. . . . . . . . 3RZHU>:@. Figure 2.1: Indication of the working areas of different energy storage systems [4].

(45) 22. 2.4. 2.4 DG Grid-Connection Characteristics. DG Grid-Connection Characteristics. Public electrical power systems operate as AC (Alternating Current) systems standardized at either 50 Hz or 60 Hz. Power is mostly generated by means of synchronous generators. The synchronous generators driven by their turbines are responsible for maintaining the frequency in the system. The frequency of the system voltage is directly proportional to the speed of the synchronous generators, i.e., p nsyn , (2.15) f= 60 where nsyn denotes the synchronous speed of the generator in revolutions per minute [rpm], p the number of pole pairs of the magnetic field circuit, and f the frequency of the generated voltage [Hz]. Due to the variety of primary energy sources/prime movers, DG can generate electricity by means of either rotating electrical machines or static electrical generators. When the primary energy is converted into mechanical energy that is used to drive electric rotating machines (synchronous or induction machines), AC power is generated. If this AC power is generated at the system frequency or close to it, the generator can be directly coupled to the grid. However, if the frequency deviates from the system frequency, power electronic interface must be used. This may occur if the primary energy sources are intermittent in nature (e.g. wind, tidal, wave) and if it is economically better to adapt the speed of the generator accordingly. DG may also generate AC power by means of a fast-rotating prime mover (e.g. a microturbine). In this way, AC power is generated at a constant, but higher frequency than that of the grid. Also in this case, an interface is required. When DC power is generated (i.e. f = 0), as with solar panels and fuel cells, an interface that converts DC to AC (at the system frequency) is a must. In this way, the DG connection to the power grid can be classified into two categories: • Direct grid-connected DG. • Indirect grid-connected DG.. 2.4.1. Direct Grid-Connected DG. Figure 2.2 shows a schematic diagram of a DG unit connected directly to the AC grid. The prime mover operates at a constant speed, and drives the generator. In general, this generation (or conversion) can be done by means of either a synchronous or an induction generator. DG units equipped with a synchronous generator For a synchronous generator, (2.15) is applicable. By controlling the prime mover, so that it operates at a constant speed, the generator can produce power at the grid frequency. This is the case in steam plants, gas turbines, combined cycle plants and co-generation plants. The difference is in the energy source that drives the prime mover..

(46) Distributed Generation. 23. . " . . . . Figure 2.2: Schematic diagram of direct grid-connected DG DG units equipped with an induction generator When the prime mover does not operate at a constant speed, an induction generator may be used. In this case, n is no longer constant and n = (1 − s)nsyn ,. (2.16). where s represents the slip. Induction generators are usually applied in small hydro-power plants and older design or small wind turbines. In this case the speed of the induction generator may vary with the turning force (moment, or torque) applied to it. In practice however, the difference between the rotational speed at peak power and at idle is very small, about 1 per cent [69]. Usually, a gearbox is used (Figure 2.3) to connect the low-speed driving shaft to the high-speed generator shaft (1200 to 1500 rpm). . & $ . " . . " %. . Figure 2.3: Schematic diagram of grid connected DG via gearbox. 2.4.2. Indirect Grid-Connected DG. A power system operates at a constant system/grid frequency. Several DG types generate electricity as DC (e.g. solar panels and fuel cells), high-frequency AC (e.g. microturbines) or AC with variable frequency (e.g. certain types of wind turbines). Therefore, an interface is necessary to connect these devices to the grid. As such, a DG unit is connected to the grid in an indirect way..

(47) 24. 2.4 DG Grid-Connection Characteristics For indirect grid-connected DG, we basically distinguish two situations: • DG generating DC. • DG generating either high-frequency AC or AC with a variable frequency. • Induction generator with power electronic converter in the rotor.. DG generating DC A DG unit with DC output is primarily characterized by static electric generation, i.e. no rotating parts are involved. Examples of these kind of DG units are fuel cells and solar panels. Figure 2.4 shows a simplified lay-out of such a plant. The primary energy sources are converted into electricity without of a rotating electrical machine. The DC output may be fluctuating and is smoothed by a capacitor before converted to AC at the grid frequency. In addition, a filter can be implemented at the output-stage of the inverter to clean the AC voltage. . # . " . ! . . $ . Figure 2.4: Interface-connected DG with DC output. DG generating high/variable-frequency AC Some DG units, such as microturbines, wind turbines and tidal power generators, use rotating electrical machines for electricity generation but are connected to the grid via power-electronic interfaces. There are two situations in which power-electronic converters are needed to interface the rotating electrical machine to the grid: • When the rotating electrical machine generates a high-frequency AC (far beyond the grid frequency). • When the primary energy sources cause the prime mover to drive the rotating electrical generator at a variable speed, leading to a variablefrequency AC. This is illustrated in Figure 2.5. The high-frequency AC, or AC with variable frequency, is rectified into DC. A capacitor is used to smooth the DC, before it is converted into grid-frequency AC. A filter can be implemented to clean the resulting AC voltage..

(48) Distributed Generation . . . 25 . . . . " . ! . . $ . Figure 2.5: Interface-connected DG with AC output Induction generator with power electronic converter in the rotor The stator windings of a variable speed induction generator can be connected directly to the grid with the rotor windings connected to (bi-directional) power electronic interface (Figure 2.6). The mechanical and electrical rotor frequencies are controllable over a certain range and the electrical stator and rotor frequency can be matched, independently of the mechanical rotor speed [28], [75]. Table 2.2 summarizes the grid connection classifications of DG. Primary energy source Prime mover. Gridfrequency AC. Variable speed. Gearbox. Rotary electrical generator DC AC to DC converter. DC DC link capacitor. DC to AC converter. Power-electronic interface. Figure 2.6: Induction generator with power electronic converter in the rotor.